NWBuck, welcome to Pond Boss.

The elimnation of the weeds as cover for the YOY (young of the year) fry are probably to blame for the reduced densities of fish, the YOY simply no longer have any cover to hide in to escape predation from the larger fish.

Probably the best way to remove the Grass Carp (GC) is with a bow and arrow.

IMO once the vegetation returns you will see higher densities of fish in your pond, your existing LMB and BG should be fine in re-populating the pond.